1 Introduction

Methyl violet, which is used in industries such as textiles, print inks and paints, is mostly used for dyeing cotton, silk, paper, bamboo, straw and leather. Methyl violet contains three aryl groups, each of which has one or two methyl groups attached to the nitrogen atom. When a pentamethyl group is present, the compound is referred to as methyl violet 2B (MV-2B) (Bonetto et al., 2015). Methyl violet is a cationic dye with carcinogenic properties, imparting an intensive violet color to water when dissolved. This reduces light transmittance and hinders photosynthesis. Cationic dyes like methyl violet 2B have significant implications for human health, causing various issues such as dryness and redness of the skin, stimulation of the digestive system, sleepiness, irritation of the mouth and throat, as well as respiratory and circulatory system diseases (Foroutan et al., 2019; Foroutan et. al., 2020). Consequently, wastewater containing MV-2B should be treated before being discharged into the receiving environment.

Adsorption is one of the most frequently used wastewater treatment methods. The adsorption of dye in wastewater is enhanced through ultrasound irradiation. Solid particles in the solution are agitated with the help of sound energy. Sono-assisted adsorption improves the mass transfer rate in the solution by drop** diffusion resistance and accelerating the chemical reaction. In sono-assisted adsorption, tiny gas bubbles form and grow until they reach a critical size. The bubbles that reach critical size collapse near the adsorbent surface. This event is known as acoustic cavitation. Microturbulence caused by acoustic cavitation reduces diffusion resistance and increases mass transfer. Sono-assisted adsorption technique has several advantages such as, a short equilibrium time compared to other conventional adsorption processes, improve the adsorption efficiency and require less amount of adsorbent (Bhowmik et al., 2019; Das et al., 2021; Deb et al., 2020).

Conventional adsorbents such as clay and zeolite are, commonly used in wastewater treatment. However, they have certain drawbacks, including low adsorption capacity, long adsorption time and difficulties in separation (Das et al., 2021). Therefore, the development of adsorbents with high adsorption capacity and easy separability is essential. To address this need, magnetic adsorbents incorporating metal oxides can be synthesized using natural clays like kaolin. Kaolin, primarily composed of kaolinite material (Al2Si2O5(OH)4), is used as an adsorbent due to its cost-effectiveness, eco-friendliness and abundance (Aragaw & Angerasa, 2020). TiO2 draws attention in wastewater treatment and adsorption of pollutants due to its chemical stability and non-toxic nature. However, it has a low surface area and is challenging to separate from water (Mustapha et al., 2021). To overcome these limitations, magnetic nanoparticles can be synthesized with kaolin and TiO2.

In the literature, sono-assisted adsorption has been employed for the removal of dyes such as eosin yellow and methyl orange using hematite polyaniline (Deb et al., 2020), eriochrome Black-T using iron oxide/polyaniline polymeric nanocomposite (Deb et al., 2021), methylene blue and methyl violet using CoFe2O4 loaded graphene oxide (Gupta et al., 2020), methylene blue using fly-ash derived nanozeolite-X (Sivalingam et al., 2019), congo red using magnetic graphene oxide (Fe3O4-GO-NH2) (Sricharoen et al., 2021), eriochrome black T using CaFe2O4/MnFe2O4 nanocomposite (Bhowmik et al. 2019), cationic dyes (methylene blue, methyl violet and Nile blue) using AC/CoFe2O4 (Foroutan et al., 2019). While significant literature exists on the utilizing of various adsorbents in sono-assisted dye adsorption, no study was found utilizing magnetic kaolin-supported titanium dioxide. Therefore, the development of a new magnetic adsorbent prepared with kaolin and TiO2 is crucial for dye removal.

This study aims to explore the sono-assisted adsorption of MV-2B. To this end, a novel magnetic adsorbent incorporating kaolin and TiO2 was synthesized and characterized using various analytical techniques including BET, SEM, EDS, FTIR, XRD, Zeta potential, and VSM. The study investigates the effects of adsorption parameters such as adsorbent amount, initial MV-2B concentration, contact time, initial pH of the solution, ionic strength, and temperature, as well as kinetics and isotherm models, on the sono-assisted adsorption of MV-2B. Furthermore, the effectiveness of sono-assisted MV-2B removal is compared with conventional adsorption methods.

2 Materials and Methods

2.1 Materials and Equipments

In the study, kaolin (a commercial product obtained from a company in Turkiye), FeSO4.7H2O (Merck), FeCl3.6H2O (Sigma Aldrich), and titanium IV butoxide (Sigma Aldrich), ethyl alcohol (Merck, %96) were used to prepare the magnetic composite. MV-2B was supplied by Isolab (C.I. 42535). All the chemicals were used without purification.

An ultrasonic bath (DSA50-SK) was used for the sono-assisted adsorption. The frequency of the ultrasonic bath was 42 kHz frequency and its volume was 1600 mL. The absorbance of the samples was measured with UV spectrophotometer (Hach, DR-2400).

2.2 Preparation of the Adsorbent and Characterization

The adsorbent used in the study was prepared by chemical coprecipitation method (Boushehrian et al., 2020; Foroutan et al., 2019). Initially, titanium IV butoxide and ethyl alcohol were mixed for 10 min. At the same time, iron II sulphate heptahydrate (FeSO4.7H2O) and iron III chloride hexahydrate (FeCl3.6H2O) were dissolved in 0.1 L distilled water at a molar ratio of 1:1, and stirred with a magnetic stirrer for 10 min. The titanium IV butoxide solution was then added to the iron solution and stirred for 5 min. After that, kaolin was added to the solution and heated to 60 °C. Then mixture was stirred for 30 min. pH of the solution was adjusted to 12 using 3 M NaOH solution. After pH adjustment, stirring was continued for one hour at 80–85 °C. The prepared adsorbent was washed with distilled water several times and filtered using filter paper (Whatmann-40). It was dried at 90 °C for 60 h. The prepared adsorbent was coded as KTF.

KTF was characterized by BET (Quantachrome Autosorb), XRD (Bruker D8 Advence), FTIR (PerkinElmer, Spectrum Two), VSM (Lake Shore 7407), Zeta potential (Malvern ZetaSizer Nano ZSP), SEM and EDS (FEI, Quanta FEG250) analyses.

2.3 Adsorption Experiments

In this study, process variables such as contact time (0 − 15 min), initial pH (3.3–9), initial concentration of MV-2B solution (20–60 mg/L), amount of adsorbent (0.1–0.6 g/100mL), amount of NaCl (0.1–0.5 g/100mL), amount of Na2SO4 (0.1–0.5 g/100mL), and temperature (22–47 °C) were investigated. These ranges were selected based on previous studies related to adsorption.

The reaction vessel was filled with 100 mL MV-2B solution with a known concentration and adsorbent. Then, the reaction vessel was placed at the center of the ultrasonic bath. The temperature of the ultrasonic bath was controlled by the addition of ice. All the experiments were repeated at least twice. The samples taken from the dye solution were centrifuged at 3500 rpm for 10 min to remove the adsorbent. The concentration of MV-2B was found using the absorbance value of the sample, recorded at a wavelength of 584 nm.

The MV-2B removal (R), was calculated using Eq. (1).

$$R, \%={\left[\left({C}_{0}-{C}_{t}\right)/{C}_{0}\right]}^{*}100$$
(1)

The adsorption capacity of the KTF at equilibrium (qe, mg/g) was calculated using Eq. 2

$${\varvec{q}}_{\varvec{e}}=\boldsymbol{ }\frac{\left({\varvec{C}}_0-{\varvec{C}}_{\varvec{e}}\right)\boldsymbol{*}{\varvec{V}}}{{\varvec{W}}}$$
(2)

where, V is the volume of the MV-2B solution (L), W is the weight of the KTF (g), C0, Ct and Ce are the concentration of MV-2B (mg/L) at initial, at any time and at equilibrium respectively.

Sono-assisted adsorption experiments were performed in the dark to test MV-2B removal by photocatalytic processes under the conditions of 0.2 g/100 mL KTF, 15 min contact time, original pH and 22 °C. There was no difference between the experiments performed in the dark and those conducted in the light. The results showed that KTF had no photocatalytic effect.

3 Results and Discussion

3.1 Characterization of the KTF

The surface structure of the KTF and the distribution of elements were investigated using SEM–EDS analyses. Figure 1a and b show the SEM and EDS analyses of the kaolin (K) and KTF. As seen in Fig. 1a, kaolin has layered and porous structure with sharply shaped. Nanoparticles are adhered to the surface of the KTF. It has a spherical soft shaped surface structure. The distribution of the elements in the K and KTF surface were given in Fig. 1b and Table 1. As shown, KTF was successfully loaded with iron and titanium.

Fig. 1
figure 1

(a) The SEM image of K and KTF, (b) EDS spectra of K and KTF

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Table 1 Results of the EDS analysis
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Figure 2a shows XRD patterns of KTF. The major diffraction peaks at (12.40°- 24.97°- 26.7°) and 20.9° are associated with the presence of kaolinite and quartz respectively (Fındık, 2022; Boushehrian et al. 2020). The reflections of maghemite (γ-Fe2O3) were found at 2θ of 35.6°, 57.39° and 63.02° corresponding to the (311), (511), (440) lattice planes of maghemite (PDF#39–1346). The diffraction peaks found for maghemite were compatible with the JCPDS No. 39–1346 (Hu et al., 2011; Zhu et al., 2010). The diffraction peaks at 2θ of 31.75° and 36.65° are compatible with TiO2 with PDF#21–1236.

Fig. 2
figure 2

(a) XRD analysis of KTF (b) FTIR spectra of KTF, and after adsorption of dye on KTF

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The FTIR analysis of the KTF and after the adsorption of MV-2B dye on KTF were given in Fig. 2b. The bands at 3620 and 3690 cm−1 are related to OH stretching (Fındık, 2022; Magdy et al., 2017). The peaks at (1120—1030—1010 cm−1), 910 cm−1 and (796—750—691 cm−1) may be due to the Si–O stretching, Al-Al–OH stretching and bending vibrations of SiO respectively (Boushehrian et al., 2020; Magdy et al., 2017). The FTIR peak at 609 cm−1 for Ti–O stretching vibration (Mustapha et al., 2021). The band at 560 cm−1 was the characteristic band of maghemite phase (Hu et al., 2011). The band at 540 cm−1 for Fe–O stretching (Chen et al., 2012). The FTIR spectra of KTF and the FTIR spectra of KTF after MV-2B adsorption have the same bands but intensity of the bands changed. This indicates that the functional groups of KTF were involved in the MV-2B adsorption (Jawad & Abdulhameed, 2020).

The adsorption/desorption analysis of N2 was used to study BET analysis of KTF. Figure 3a shows the adsorption/desorption isotherm. As seen, it is a lineer type isotherm. The pore size distribution plot was given in Fig. 3b. The pore volume and mean pore radius of the KTF were determined as 0.072 cm3/g and 13.4 A° respectively. The BET specific surface area for KTF were determined as 65.279 m2/g. On the other hand, BET surface area and pore volume of kaolin was determined as 1.528 m2/g and 0.0336 cm3/g. These findings indicate that KTF exhibits a significantly higher surface area and pore volume compared to kaolin.

Fig. 3
figure 3

(a) N2 adsorption/ desorption isotherm plot, (b) pore size distribution plot of KTF

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Figure 4 shows the VSM analysis. The magnetic saturation value of KTF was determined to be 8.6 emu/g at room temperature in the magnetic field of ± 20,000 Qe. The magnetization curve of KTF exhibits superparamagnetic behavior with zero coercivity and remanence at room temperature (Nicola et al., 2020). The absence of a hysteresis loop indicates that the samples are superparamagnetic (Rossatto et al. 2020). KTF can be separated from the MV-2B solution with a magnet.

Fig. 4
figure 4

VSM analysis of KTF at room temperature

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3.2 Effect of KTF Amount

Before investigating the adsorbent amount, experiments were performed using only ultrasound (without adsorbent). The results showed that there was no removal of MV-2B using only ultrasound at the end of 15 min. The effect of KTF amount on sono-assisted MV-2B removal was investigated at 20 mg/L initial MV-2B concentration, original pH and 15 min contact time. Figure 5a shows the results. The removal of MV-2B increased with increasing KTF amount from 0.1 g/100mL to 0.2 g/100mL. After 0.2 g/100mL, removal rate decreased. The maximum MV-2B removal was found to be 85.6% in the KTF amount of 0.2 g/100mL.

Fig. 5
figure 5

(a) Effect of KTF amount (initial MV-2B concentration: 20 mg/L, temperature: 22 °C, initial pH: original, contact time: 15 min) (b) Effect of initial pH (initial MV-2B concentration: 20 mg/L, temperature: 22 °C, KTF amount: 0.2 g/100mL, contact time: 15 min), (c) Zeta potential analysis of KTF (d) Effect of temperature (initial MV-2B concentration: 20 mg/L, initial pH: original, KTF amount: 0.2 g/100mL, contact time: 15 min)

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Increasing the amount of adsorbent results in a higher surface area and more adsorption sites (Bhowmik et al. 2019). However, after reaching the optimal amount of adsorbent, the removal rate remains nearly constant. This is because the agglomeration of particles reduces the surface area, thereby decreasing the adsorption rate. Therefore, using an excess amount of adsorbent is unnecessary. Similar results have been reported in the literature for the adsorption of methyl violet (Bonetto et al., 2015; Duan et al., 2012; Pooladi et al. 2021).

3.3 Effect of Initial pH and Temperature

The effect of the initial pH of the MV-2B solution was examined within the range of 3.3 to 9. The original pH of the MV-2B solution was 5.3, meaning no initial pH adjustment was necessary. The pH of the MV-2B solution was adjusted before starting the experiment and was not controlled during the experiment. As shown in Fig. 5b, the removal rates of MV-2B were found to be 85.4%, 85.6%, 84.4%, 84.5%, and 84.2% at solution pH levels of 3.3, 5.3, 7.4, 8, and 9, respectively. There was no significant change in the removal of MV-2B within the studied pH range. In the literature, Chung et al. (2022) obtained similar results for the adsorption of cationic dye methylene blue. They observed small differences in the removal of methylene blue within a pH range of 2–10. According to the authors, the nearly constant adsorption efficiency at different pHs levels indicated the stability of the adsorbent.

One of the most crucial parameters for determining the surface properties of an adsorbent is its surface charge, as it can influence the interaction between the adsorbent surface and contaminants (Boushehrian et al., 2020). The amount of surface charge can be determined by the zero isoelectric point (pHZPC) (Foroutan et al., 2020). The pHZPC is the pH value where positive and negative charges are equal on the surface of a material (Bonetto et al., 2015). Figure 3c shows the results of the Zeta potential analysis of KTF. As shown, the pHZPC value for KTF was 3.4. When the pH value of the solution is below the pHZPC value, the adsorbent has a positive charge, when the pH value is above the pHZPC, the adsorbent surface has a negative charge (Boushehrian et al., 2020; Foroutan et al., 2019).

At pH 3.3 (pH < pHZPC), there is an electrostatic repulsion force between the KTF surface and MV-2B. The adsorption of MV-2B may be due to the strong π-π interactions between the surface of KTF and MV-2B molecules (Foroutan et al., 2020; Mojarad et al., 2022). On the other hand, when the pH of the MV-2B solution is greater than the pHZPC (pH ˃ 3.4), the surface of KTF is negatively charged. MV-2B is a cationic dye, so the adsorption of MV-2B occurs due to electrostatic interaction between the negatively charged KTF surface and positively charged MV-2B molecules (Foroutan et al., 2019; Mojarad et al., 2022).

The effect of temperature on the adsorption of MV-2B was examined at 22, 30, 40 and 47 °C at 20 mg/L initial MV-2B concentration, original pH, 0.2 g/100mL adsorbent amount and 15 min contact time. Figure 5d shows the results. MV-2B removal was found to be 85.6%, 85.3%, 83% and 85% at 22, 30, 40 and 47°C respectively. The optimum temperature was 22°C. There was nearly constant removal rate at the studied temperature range. The stable performance efficiency was observed as pH effect. In literature, Chung et al. (2022) obtained similar results.

3.4 Effect of Contact Time and Initial MV-2B Concentration

Figure 6a shows the contact time effect on the sono-assisted adsorption of MV-2B at 20 mg/L initial concentration, 0.2 g/100mL KTF amount, 22 °C and original pH. The sono-assisted removal of MV-2B was very fast. 80.3% removal was observed within 2 min. In literature, Deb et al. (2021) obtained similar result for the sono-assisted adsorption of eriochrome Black-T. The faster removal of dye at the beginning was due to the presence of large number of empty active sites on the adsorbent surface.

Fig. 6
figure 6

(a) Effect of contact time (initial MV-2B concentration: 20 mg/L, pH: original, KTF amount: 0.2g/100mL, temperature: 22°C) (b) Effect of initial dye concentration on the removal of MV-2B and adsorption capacity of KTF (pH: original, temperature: 22 °C, KTF amount: 0.2g/100mL, contact time: 15 min) (c) Effect of ionic strength on the removal of MV-2B (initial MV-2B concentration: 20 mg/L, pH: original, temperature: 22 °C, KTF amount: 0.2 g/100mL, contact time: 15 min)

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The effects of initial dye concentration on sono-assisted MV-2B removal were investigated at 22 °C, original pH and KTF amount of 0.2 g/100mL. Figure 6b shows the effect of initial dye concentration on the removal of MV-2B and adsorption capacity of KTF. The removals of MV-2B (at 15 min contact time) were found to be 85.6%, 85.1%, 83.2%, 83.2% and 83.5% at 20, 30, 40, 50 and 60 mg/L initial concentrations, respectively. Due to the presence of sufficient active sites on the adsorbent at a lower dye concentration, dye molecules quickly fill the binding sites (Bhowmik et al. 2019). According to Foroutan et al. (2019), sono-assisted adsorption of methyl violet decreases with increasing initial concentration due to the saturation of active sites on the adsorbent.

As seen from Fig. 6b, adsorption capacity of the KTF increased from 8.8 mg/g to 26.3 mg/g with increasing initial MV-2B concentration from 20 to 60 mg/L. The qe value increases with increasing methyl violet concentration due to increase in dye molecules per adsorbent mass unit. At high dye concentration, there is a big concentration gradient, so mass transfer rate increases (Foroutan et al., 2020). Hayoune et al. (2024) reported that adsorbed dye amount increases with increasing initial dye concentration. As the dye concentration increases, the diffusion of dye molecules in the pores increases with the increase of the driving force.

3.5 Effect of Ionic Strength

To investigate the effect of ionic strength on the sono-assisted removal of MV-2B, NaCl and Na2SO4 were used under the following conditions: an initial MV-2B concentration of 20 mg/L, a temperature of 22 °C, KTF amount of 0.2 g/100mL, original pH and a contact time of 15 min. As shown in Fig. 6c, the sono-assisted removal of MV-2B decreased with increasing the concentrations of NaCl and Na2SO4. This decrease can be attributed to the competition between the Na+ and the MV-2B molecules (Foroutan et al., 2021). Sodium ions (Na+) interact with the active sites and the negatively charged functional groups on the adsorber surface through electrostatic interaction, thereby reducing the adsorption efficiency by creating electrostatic repulsive forces between the adsorber surface and MV-2B molecules due to the co-electric charge of sodium and MV-2B molecules (Foroutan et al., 2020, 2021).

3.6 Comparison of Sono-assisted Adsorption with Conventional Adsorption

Figure 7 shows the removal of MV-2B using stirring, shaking and sono-assisted adsorption at 20 mg/L initial dye concentration, original pH, 15 min contact time and 0.2 g/100mL KTF amount. It was observed that the sono-assisted removal of MV-2B was 85.6%. On the other hand, the removal of MV-2B found as 37% and 60.5% with shaking and stirring respectively. There are similar results in literature. The sono-assisted adsorption of dyes such as malachite green, eosin yellow, methyl orange and eriochrome Black-T was higher than the other adsorption methods (Deb et. al., 2020; Deb et al., 2021; Das et al., 2021). Sono-assisted adsorption intensifies the mass transfer of dye onto adsorbent surface by reducing the diffusion resistance. Thus sound wave improves the adsorption efficiency, reduces adsorbent amount and equilibrium time (Bhowmik et al. 2019; Das et al., 2021). In one study, Hayoune et al. (2024) reported that sono-assisted adsorption of methylene blue using magnetic Algerian Halloysite clay was higher than the classic adsorption. In another study, Chen and Huang (2021) investigated traditional and sono assisted adsorption of methylene blue using chromium-based metal organic frameworks derived from waste PET bottles. According to their results, sono-assisted adsorption rate of methylene blue was higher than the traditional adsorption. In sono-assisted adsorption, the adsorption capacity of the adsorbent increases as the probability of collision between adsorbent and adsorbate increases.

Fig. 7
figure 7

Comparison of adsorption methods (initial concentration: 20 mg/L, pH: original, temperature: 22 °C, KTF amount: 0.2 g/100mL, contact time: 15 min)

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3.7 Adsorption Isotherms

The common isotherm models such as Langmuir, Freundlich and Temkin were applied to analyze the sono-assisted adsorption of MV-2B onto KTF in the range of 20 − 60 mg/L initial concentrations while the other factors were kept constant (temperature: 22 °C, initial pH: original, KTF amount: 0.2 g/100mL).

The Langmuir isotherm model assumes that the adsorbent surface is homogeneous, the adsorbate covers the adsorbent surface in a monolayer and active sites on the adsorbent are identical and energetically equivalent (Bonetto et al., 2015). Freundlich isotherm assumes multilayer adsorption, and the adsorbent has a heterogeneous surface with different sorption energies (Chung et al., 2022; Foroutan et al., 2019). According to the Temkin isotherm, the adsorbent surface is heterogeneous and the adsorbate is distributed in a monolayer over the active sites. It is also assumed that the distribution of binding energy is uniform (Bhowmik et al. 2019).

The lineer form of the Langmuir, Freundlich, and Temkin models are given in Eqs. 3, 4 and 5 respectively (Bonetto et al., 2015; Boushehrian et al., 2020; Deb et al., 2020).

$$\frac{C_e }{q_e}=\frac{C_e}{q_{max}}+\frac{1}{q_{max} {^K}{_L}}$$
(3)
$$lnq_e=lnk_f+ \frac{1}{n}lnC_e$$
(4)
$${q}_{e}={\beta }_{1}1n{K}_{T}+{\beta }_{1}1n{C}_{e}$$
(5)

where qmax is the adsorption capacity (mg/g), KL is the adsorption energy (L/mg), kf is adsorption capacity of the adsorbent, n is the Freundlich constants, KT (L/mg) and β1 are both known as Temkin isotherm constants.

The calculated values of the isotherm models for sono-assisted adsorption of MV-2B are given in Table 2. The Freundlich adsorption isotherm model for sono-assisted adsorption of MV-2B is shown in Fig. 8. The R2 values found for the studied isotherm models were as follows: 0.47 for Langmuir, 0.985 for Freundlich and 0.923 for Temkin. According to R2 values, sono-assisted adsorption of MV-2B on KTF was described by Freundlich isotherm model. The value of “n” in the Freundlich isotherm determines whether the adsorption process is linear (n = 1), physical (n > 1) or chemical (n < 1) (Boushehrian et al., 2020). In this study, value of n was found to be 1.15. It showed that the adsorption of MV-2B onto KTF is physical and desirable.

Table 2 Isotherm parameters for the sono-assisted adsorption of MV-2B (pH: original, KTF amount: 0.2 g/100 mL, temperature: 22 °C)
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Fig. 8
figure 8

Freundlich isotherm for MV-2B onto KTF (pH: original, temperature: 22 °C, KTF amount: 0.2 g/100 mL)

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3.8 Kinetic Analysis of Sono-assisted Adsorption of MV-2B onto KTF

The sono-assisted adsorption kinetic analysis of MV-2B onto KTF were performed using common kinetic models such as pseudo first order (Ps.FO), and pseudo second order (Ps.SO) models. Table 3 presents the linear forms of the aforementioned kinetic models (Boushehrian et al., 2020; Das et al., 2021) and kinetic parameters obtained from experimental data. The value of the regression coefficient (R2) gives information about the agreement between the qe,cal values and the qe,exp values obtained from the experiments. A relatively higher R2 value shows that the kinetic model is suitable for the adsorption process. The results of the Ps.SO model obtained for the sono-assisted adsorption of MV-2B are shown in Fig. 9.

Table 3 Kinetic models constants for sono-assisted adsorption of MV-2B using the adsorbent KTF (KTF amount: 0.2 g/100 mL, pH: original, temperature: 22 °C)
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Fig. 9
figure 9

Ps.SO model for the sono-assisted adsorption of MV-2B onto KTF

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As shown in Table 3, the correlation coefficient of the Ps.SO model (R2: 0.999–1) was higher than that of the Ps.FO (R2: 0.6137–0.9061). In addition, qe,exp and qe,cal values are close to each other in the Ps.SO model whereas for the Ps.FO model there was a high deviation between calculated and experimental qe values. This result shows that the Ps.SO was the best model in describing the kinetics of KTF toward MV-2B. According to this result, the uptake of MV-2B dye using the KTF was done chemically, which includes the electrostatic interaction between negative charges on the surface of KTF and positive charge of MV-2B. It means that the adsorption mechanism is regulated by chemisorption (Deb et al., 2020; Jethave et al., 2021; Mojarad et al., 2022).

3.9 The Reusability of KTF

The reusability of KTF was tested under optimal conditions: 20 mg/L initial dye concentration, original pH, 15 min contact time and 0.2 g/100 mL KTF amount. The used adsorbent was separated from MV-2B solution, washed with distilled water, dried at room temperature, and then reused as an adsorbent in the sono-assisted adsorption process of MV-2B. Figure 10 illustrates the removal efficiency of KTF after three cycles of reuse. The removal of MV-2B decreased with an increasing number of cycles. After the third cycle, the removal of MV-2B was 69%, while 85.6% removal was achieved in the first cycle. This decrease in adsorption efficiency could be attributed to the saturation and damage of active sites on the composite surface (Boushehrian et al., 2020). However, the removal rate of MV-2B after reuse may increase with chemical or thermal regenaration of the adsorbent. In a study conducted by Gupta et al. (2020), the reusability of thermally regenerated and non-generated adsorbents was examined. The removal performance of the thermally regenerated adsorbent was higher than that of the non-regenerated one after the fourth cycle.

Fig. 10
figure 10

Removal efficiency of MV-2B using KTF for three cycles (initial MV-2B concentration: 20 mg/L, pH: original, KTF amount: 0.2 g/100 mL, temperature: 22 °C, contact time: 15 min)

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3.9.1 Comparison of KTF with Other Adsorbents

The adsorption capacity of KTF for the removal of MV-2B was compared with that of other adsorbents reported in the literature (Table 4). The maximum adsorption capacity of KTF was found to be 131.58 mg/g for sono-assisted adsorption of MV-2B. Among the different adsorbents studied for the adsorption of methyl violet, KTF exhibited a higher adsorption capacity. However, despite its higher adsorption capacity and shorter contact time, it was observed that a higher amount of KTF led to a lower removal efficiency compared the other adsorbents in literature. Improving the structure of KTF is necessary to enhance dye removal efficiency. Numerous studies have been conducted to develop a novel adsorbent with high adsorption capacity and removal efficiency. Therefore, the findings of this study will provide valuable data for future researchers.

Table 4 Comparison of the KTF with other adsorbents for the removal of methyl violet
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4 Conclusion

In this study, sono-assisted adsorption of methyl violet 2B (MV-2B) was investigated. A magnetic adsorbent (KTF) composed of kaolin and TiO2 was prepared by co-precipitation method. The structure of KTF was characterized by analyzes such as BET, FTIR, SEM, EDS, XRD, Zeta potential and VSM. EDS results showed that kaolin successfully loaded with iron and titanium. XRD analysis confirmed the formation of maghemite (γ-Fe2O3) and TiO2 in KTF. BET surface area and mean pore diameter of the KTF were 65.279 m2/g and 13.4 A° respectively. According to VSM analysis, KTF showed superparamagnetic behavior. The effects of the contact time, initial MV-2B concentration, KTF amount, temperature, ionic strength and initial pH of the solution, on the adsorption of MV-2B were studied. Sono-assisted adsorption of MV-2B was found to be 85.6% under the conditions at a KTF amount of 0.2 g/100 mL, an initial dye concentration of 20 mg/L, a contact time of 15 min at a temperature of 22 °C and original pH. The results showed that sono-assisted adsorption of MV-2B was described by pseudo second order model and Freundlich isotherm model.